Hi Everyone,
I apologize in advance for asking for help for something seemingly minor but...

My sister gave me her old iPhone 3GS, the screen is broken...I thought I'd try to jailbreak it then unlock it so I can use my T-Mobile sim with the phone but I can't seem to figure out how to do this. Any help would be awesome.

I did restore it to iPhone OS 4.3.1 which may or may not have been a mistake but I wanted to update it first.

It also has Baseband 05.16.02-6.4_M3S2.

I've been trying all day and can't seem to figure it out! I can't seem to restore an older operating system either, I figured that might be the easiest thing because untethered jailbreak of 4.3.1 does not seem to exist yet, but untethered is not important.

Would I put the new sim card in before I do all the jailbreaking?

Thanks in advance for any help you can offer.

Quick update...

I was able to successfully jailbreak my iPhone 3GS. Like I said I was able to downgrade the OS to 4.1 after I had inadvertently restored to 4.3.1. I did this by saving my SHSH blobs and then using Pwnage Tool and the instructions here...iOS 4.1 Jailbreak For iPhone 3GS: Easy 7 Steps Tutorial | areacellphone.com

Now although this worked and I now have Cydia on my iPhone it still won't unlock, using the ultrasn0w application...I downloaded it and installed it but it still won't work...any suggestions??? I have noticed that on the main Cydia page it says "This device is in the pending TSS queue" so I'm hoping that when that goes away I will be able to use my other sim card...any ideas?

thanks in advance.

What baseband does the phone currently have? If it is 5.16.00, 5.15.04 or 5.14.02 then ultrasn0w doesn't work with these basebands.

You have two choices: wait for an unlock from the iPhone dev team that works on these baseband versions. Or update (can't be ever reversed) to to the iPad baseband 6.15 and use ultrasn0w to unlock.